1. Field of the Invention
The present invention relates to a semiconductor memory device having a reference cell that serves as a reference during data read.
2. Description of the Related Art
In recent years attention has been given to a nonvolatile memory configured as an array of memory cells, each memory cell formed by connection of a variable resistor to the intersection of a word line and a bit line.
Known as this kind of nonvolatile memory are the likes of a PCRAM (Phase-Change Random Access Memory) using a chalcogenide element in the variable resistor; a ReRAM (Resistance Random Access Memory) using a transition metal oxide element; and what is known as a CBRAM in which metal cations are deposited to form an inter-electrode bridge (conducting bridge) and resistance is varied by ionizing the deposited metal to destroy the bridge. The feature of these variable-resistance memories lies in the fact that variations in resistance are stored as information.
In PCRAM, resistance of a chalcogenide element is varied by causing phase change of the element into a crystalline state or an amorphous state, through control of a process effected on the element that ranges from heating through cooling depending on the size and form, for example, width of a current/voltage pulse applied to the element (refer to Patent Document 1: JP2002-541613T). ReRAMs can be of bipolar type and unipolar type. In the case of a bipolar type, resistance of a transition metal oxide element is controlled by the direction of a current/voltage pulse applied to the element. On the other hand, in the case of a unipolar type, resistance of the transition metal oxide element is controlled by the size, width and so on of the current/voltage pulse applied to the element.
Reading of information from these variable-resistance memories is effected by passing a read current through a memory cell and a reference cell, converting their respective resistances into voltages, then comparing the voltages of the memory cell and reference cell.
In the above-described nonvolatile memories, the parasitic resistance of word lines and bit lines differs according to cell position within the array and the drop in voltage caused thereby means that it is sometimes not possible to accurately read the voltage of the memory cell and reference cell. As a result, there is a possibility that the actual data stored in a variable resistor may be misread.